THIS invention relates to a rotary boring bit and to a rotary borer incorporating the bit.
Rotary coal borers are used to drill holes in coal mines. The holes may be in the roof, to be used for roof bolting to support the roof, or they may be used for drilling blast holes in the coal body itself. A common form of coal boring bit or insert is made from or includes hardmetal such as cemented tungsten carbide. The bit is in the form of a generally rectangular blade with one of its long edges being upwardly inclined from its short edges to provide a gabled cutting edge with the apex of the cutting edge being in register with the axis of rotation of the steel borer body. The bit is partially embedded in the body and is generally fixed to it by brazing with its cutting edge standing proud of the borer body material in the drilling direction of the borer.
In addition to the cutting edge, the side edges on known bits project radially from the borer body and have what is known as a clearance angle on the body gauge which is intended to minimize jamming of the borer during drilling.
The bit of a borer of the above type rotates during drilling on a pressure point at the apex of the cutting edge, with little cutting action taking place at that point. This inhibits the penetration rate of the borer. A further disadvantage of the known borers is that the clearance tapers of the side edges significantly reduce the length of the reaming edges of the bit and so reduce the resistance of these short edges to wear. As a result the borers can become under gauge prematurely and have to be discarded at an early stage.
According to one aspect of the invention there is provided a rotary boring bit, typically for coal drilling applications, which is in the form of a hardmetal blade and which includes a cutting edge having an outwardly projecting gable shaped cutting formation on either side of the intended axis of rotation of the blade, a base which is spaced on the axis from the cutting edge and parallel side edges which extend between the cutting edge and base.
For improved penetration rates, it is preferred that the gable shaped cutting formations have apices which are unequally spaced from the axis of rotation of the bit, and that the sloping gable side edges be inclined at an angle in the range 30xc2x0 to 34xc2x0, preferably about 32xc2x0, relative to a plane normal to the axis of rotation of the bit. The apices may however lie in a common plane normal to the axis of rotation of the bit.
The cutting edge of the bit may be sharpened, typically at an angle of about 17xc2x0 to a plane normal to the axis of rotation, in opposite directions on opposite sides of the axis of rotation of the bit. Sharpening may take place, on each side of the axis of rotation of the bit, in a direction away from the direction of movement of that side during rotary drilling. Similarly, the parallel side edges of the bit may be sharpened in opposite directions, with each side once again being sharpened in a direction away from the direction of movement of that side during rotary drilling.
According to another aspect of the invention there is provided a rotary borer comprising a substantially cylindrical body and a bit, as summarised above, fixed diametrically in the body with its cutting edge projecting from the body in the drilling direction of the borer and its side edges projecting radially from the side of the body.